A single dose of antibiotics given immediately after a deer-tick bite is effective in preventing Lyme disease, researchers reported yesterday, though they hastened to caution that overuse of the medication could make the disease resistant to drugs.

A dose of the antibiotic doxycycline prevented the disease in 87 percent of patients, according to the study of patients in Westchester County, N.Y. The report, to be published July 12 in the New England Journal of Medicine, was released yesterday because of the onset of summer and the accompanying fear of Lyme disease.

An editorial accompanying the New York study noted that the risk of Lyme disease from an untreated tick bite is already very low -- just 3.2 percent, which would mean treating 40 people with bites to prevent a single case of the disease. The treatment can increase the risk of bacterial resistance and side effects.

Only nymphal stage bugs filled with blood pose a risk, and the study's lead investigator, Dr. Robert Nadelman of New York Medical College, said it might make sense to treat people if they were bitten by such a tick in an area where Lyme disease is common. Deer ticks go through three stages -- larval stage ticks have six legs, while nymphal and adult ticks have eight. Nymphal ticks exist in most places only from May through July.

Dr. Eugene Shapiro of Yale University School of Medicine urged clinicians to avoid giving the antibiotic to pregnant women and children younger than 8 years old, and to limit its use to people in high-risk areas who have had a tick attached for a day or two, since most infections seem to come from ticks that embed themselves rather than biting and moving on.

"There are still many thousands of people who would fall into that category, " Nadelman said. On the other hand, many people infected with Lyme disease never see the tiny tick in the first place, so would not have a chance to benefit.

About 15,000 cases of Lyme disease are reported annually in the United States, mostly in the Northeast from Maine to Maryland; the Midwest in Wisconsin and Minnesota, and the West in Northern California and Oregon.

The disease is spread by deer ticks and causes symptoms such as chronic joint pain and memory problems.

A second study, led by Dr. Mark Klempner of Boston University School of Medicine, addressed the treatment of people who had Lyme disease and later developed symptoms such as fatigue, aches and memory loss. He found that two of several available antibiotic treatments did not help patients with these persistent symptoms.

The studies -- particularly Klempner's -- added to a sometimes bitter debate that has raged between researchers and patient advocates who feel some doctors and insurance companies are reluctant to treat Lyme disease aggressively or recognize its severity.

Klempner's study reported that for patients with long-term chronic symptoms not alleviated by an initial course of antibiotics, the two treatments he studied were no better than a placebo.

Dr. Sam Donta, a professor of medicine at Boston University School of Medicine, said that the study should be the first of a series of antibiotic trials. He questioned why the study was designed to test only one course of antibiotics.

"Does that tell us that antibiotics don't work for chronic Lyme disease or that the right antibiotic wasn't used for the right amount of time?" he said.